Referring to FIG. 11, a prior art optical tracking error detecting system will be described hereinafter. Reference numeral 1 designates a semiconductor laser element (laser diode) used as a laser light source. When a diverged laser beam therefrom passes through a collimator lens 2, it is made as a parallel beam. This parallel beam is deflected by 90.degree. by a beam splitter 3 and then introduced to an objective lens 4. A converged beam from this objective lens 4 is irradiated on an optical disc provided as an optical record medium and is focused thereon. A beam emitted from this optical disc 5, that is, a reflected beam is introduced again to the objective lens 4 in which it is collimated to a parallel beam, which passes through the beam splitter 3 to a two-divided photo detector 6. As used herein, the term "two-divided photo detector" will be used to refer to a pair of photo detectors of like characteristics in side-by-side relationship.
This two-divided photo detector 6 is formed of two photo detection sections 6A and 6B as shown in FIG. 12. When a circular spot SP formed by the parallel beam from the objective lens 4 is located over the two photo detection sections 6A and 6B by just half the converged beam from the objective lens 4 scans just the center of the track on the optical disc 5. Accordingly, if both the detected outputs from these two photo detection sections 6A and 6B are supplied to a differential amplifier 7 and thereby a difference therebetween is calculated, a tracking error signal is developed at an output terminal 8.
In such prior art optical type head tracking error detecting system, however, as shown by a broken line in FIG. 11, if the objective lens 4 is moved in parallel relation to the optical disc 5, since the two-divided photo detector 6 is fixed, the position of the spot SP focused thereon is displaced as shown by a broken line in FIG. 12, producing a DC fluctuated component in the tracking error signal from the output terminal 8. In other words, if the position of the objective lens 4 is displaced as shown in FIG. 4A, the DC component of the tracking error signal is fluctuated as shown in FIG. 4B.
Further, if a radial skew takes place in the optical disc 5 as shown by a broken line in FIG. 13, since the two-divided photo detector 6 is fixed similarly, the position of the spot SP focused thereon is displaced as shown by a broken line in FIG. 14, producing a DC fluctuated component in the tracking error signal from the output terminal 8. Accordingly, also in this case, the DC component of the tracking error signal is fluctuated as shown in FIG. 4B.
In view of such aspect, the present invention is intended to provide a tracking error detecting system of an optical type head which can remove a DC fluctuation in a tracking error signal caused by the lateral movement of an objective lens and a radial skew of an optical record medium.